As a cannot find the answer in the products data sheets. Can you tell me if the 8 input channels of the TC-08 are isolated against each other?

We need to sample temperatures from a metal pipe where we apply 8 thermocouples type-k to the same pipe which then is heated up. Most dataloggers we found so far, have no isolation of their input channels against each other which leads to interference of the temperature measurements if there is a common ground.

Just to add to this, the inputs are differential with a 7.5V common mode voltage. To put this another way as long as there is no more than ±7.5V between the thermocouples you will be OK. As you mention they are connected to the same pipe it seems unlikely there would be more than a few mV voltage drop along the pipe so it should be OK.